Metabolism in general and bone metabolism in particular depend on the maintenance of a normal concentration of calcium in the plasma. Maintenance of a normocalcemia is in turn strongly dependent on the proper regulation of calcium absorption in the intestine. For this reason, an understanding of the factors involved in the regulation of calcium absorption, their mechanism of action and their quantitative significance in the maintenance of calcium homeostasis is essential. The objective of this proposal is to examine active calcium transport in the intestine during lactation with the specific aims of 1) determining the roles of alkaline phosphatase, CA-dependent ATPase and calcium binding protein (CaBP) in the nonvitamin D mediated calcium transport which occurs during lactation, 2) testing the hypothesis that intestinal hypertrophy is partially responsible for the increased calcium absorption observed during lactation and 3) testing the hypothesis that prolactin, independent of vitamin D, is directly involved in the modulation of active calcium transport in the intestine during lactation. Vitamin D replete and deficient, lactating and nonlactating, rats will be used. Active calcium transport will be determined using the everted gut sac technique. Brush border and basolateral membranes from the intestinal mucosa will be isolated and the specific activities of alkaline phosphatase and calcium-dependent ATPase measured. The mucosal concentration of CaBP will be determined using a radial immunodiffusion assay. Mucosal hypertrophy will be induced by small bowel resection and its effect on nonvitamin D mediated active calcium transport established. THe presence or absence of specific prolactin receptors in the intestinal mucosa will be evaluated using a microsomal membrane fraction and 125I labeled prolactin. The effect of exogenous prolactin on active calcium transport in the intestine of the vitamin D deficient animal in the plasma of the lactating animal through the use of bromocriptine or hypophysectomy.